See our Guidance notes, page 6, paragraph 2. When filling in your forms people want the information they need there and then. Put yourself in the form users position and think about which questions they might have a problem with. Provide examples of the sorts of answers you are looking for, and where appropriate, provide guidance notes as near as possible to the question they relate to (i.e. in the actual form).

In paper forms the specific sub-questions cannot be hidden from the user when they are not relevant to them - but in interactive forms this is relatively easy to do. Use progressive revealing as much as possible in your form design to shield your users from questions they do not need to see.